1. Field of the invention
The invention concerns a gate turn off thyristor which comprises a pellet of semiconductor material incorporating an N type cathode layer divided into separate oblong domains with major and minor principal axes intersecting at a center and defining respective unit cells, a P type gate layer which has a free surface between the cathode domains, a base layer comprising a lightly doped N type first sublayer forming a junction with the gate layer and a heavily doped N type second sublayer forming a junction with a P type anode layer, and respective metal contact layers deposited on the free surfaces of the cathode domains, of the gate layer round the cathode domains and of the anode layer, the heavily doped N type second sublayer passing in a localized way through the anode layer to form anode short-circuit areas in contact with the anode metal layer.
2. Description of the prior art
Thyristors commonly use an NPNP structure; they block a forward voltage if the current injected via the gate is below a threshold. When the gate current exceeds this threshold the thyristor conducts and continues to conduct even if the gate current is interrupted.
Gate turn off (GTO) thyristors (see for example documents FR-A-2565409 , FR-A-2610402) can be returned to the blocking (or off) state by draining charge through the gate. This blocking capability is obtained in structures in which the junction boundaries between the N type cathode layer and the P type gate layer are very large in relation to the surfaces of the junctions, to favor draining of charge by the gate below the cathode layer. An arrangement that is frequently used has the cathode layer disposed in oblong islets or domains the major axis of which is much longer than the minor axis.
In many applications there is no need for the thyristor to be able to withstand reverse voltages of comparable magnitude to the forward voltages for which it is designed; a diode is frequently connected in parallel with the thyristor to conduct in the opposite direction in order to damp out additional blocking current in inductive loads. "Asymmetric" thyristors (so called because of the asymmetry of their breakdown voltages in the forward and reverse directions) have been developed (see, for example, document FR-A-2514558). In practice the asymmetry is obtained by dividing the N type base layer into a lightly doped first sublayer and a heavily doped second sublayer; this arrangement substantially doubles the breakdown field in the forward direction with the result that the thickness of the base layer can be significantly reduced; losses essentially occur in the lightly doped N type sublayer. In particular, the asymmetric structure reduces switching losses when the device is turned off.
Both symmetric and asymmetric thyristors have been provided with anode short-circuits in the form of areas in which the N type base layer (or the heavily doped N type second sublayer in the case of asymmetric thyristors) extends through the anode layer to the anode metalization.
The effect of these anode short-circuits is to increase the speed at which electrical charge is evacuated during the blocking phase, especially at the end of this phase, and consequently to reduce the turn off losses.
On the other hand, thyristors with anode short-circuits feature an increased turn on current and therefore increased turn on losses and control power losses.
Conventionally the anode short-circuit areas are circular in shape and are disposed facing the center of the cells (defined by the cathode domains).
An object of the invention is to create an asymmetric thyristor with anode short-circuits featuring a reduced turn on current and balanced turn off function through avoiding concentration of current in certain cells during the turn off phase.